1. Field of the Invention
The present invention relates to a sheet conveying apparatus which has a position detecting apparatus which detects an end part position of a sheet, and an image forming apparatus having this sheet conveying apparatus.
2. Description of the Related Art
A conventional image forming apparatus pulls out sheets on which images are formed, one by one from a sheet cassette by means of feeding rollers to feed to the image forming portion. Therefore, there are cases where sheets to be fed (skewed and fed) are conveyed in an inclined state with respect to a sheet conveying direction due to the difference between outer diameters of feeding rollers or the difference between feeding speeds produced by friction of feeding rollers, or the influence of the sliding resistance between sheets and a conveying guide which conveys sheets.
When sheets are skewed and fed, if, for example, a toner image on a photosensitive drum is transferred onto a sheet, the toner image is printed in a state where the image is inclined with respect to the sheet. Hence, the image forming apparatus has a shutter member in a pair of registration rollers to correct skew feeding of sheets (hereinafter, also “skew feeding correction”), and aligns the front end of a sheet to be adjusted in a direction orthogonal to the conveying direction to convey to the image forming portion.
However, with the above skew feeding which is performed by having the front end of a sheet hit the shutter member, although it is possible to perform skew feeding correction of conveying sheets virtually parallel to the sheet conveying direction, it is not possible to correct the position of the sheet misaligned in a direction orthogonal to the sheet conveying direction.
Further, when, for example, duplex printing of sheets is performed, an image is formed on the first face, then the sheet is reversed and an image is formed on the second face, and therefore a conveying route for an image to be formed on the second face is longer than the first face. Therefore, the second face is susceptible to the influence of various rollers and conveying guide. By this means, sheets are likely to be skewed and fed, or misaligned. Further, sheets on which toner images are fixed in a fixing portion are contracted due to heat and pressure of the fixing portion, and is smaller upon printing of the second face than upon printing of the first face. By this means, there are cases where the end part position of a sheet is changed in a direction orthogonal to the conveying direction.
By contrast with this, a position detecting apparatus is discussed which corrects position misalignment of sheets by detecting an end part position of a sheet in a direction orthogonal to the conveying direction (Japanese Patent Laid-Open No. 2000-335010 and FIG. 1 of Japanese Patent Laid-Open No. 62-40475). With the position detecting apparatus discussed in Japanese Patent Laid-Open No. 2000-335010, line sensors are arranged in a line to detect end part positions of the minimum size to maximum size of sheets to be conveyed, and detect a side edge portion of a sheet based on a light blocked state by irradiating one face of the sheet with light. According to Japanese Patent Laid-Open No. 62-40475, a plurality of sensors is closely arranged in a width direction to detect an end part position of a sheet.
However, the position detecting apparatus discussed in Japanese Patent Laid-Open No. 2000-335010 detects side edge portions of conveyable sheets of all sizes, and therefore requires line sensors having a length which enables detection of end portions of all sizes. Therefore, there is a problem that cost of sensors becomes high, and, as a result, cost of the entire apparatus becomes high. According to Japanese Patent Laid-Open No. 62-40475, while a detecting portion which is provided with a plurality of sensors aligned in a width direction is moved according to a sheet size and therefore the number of sensors is comparatively a little, more sensors are necessary to precisely detect positions of sheets, and therefore cost increases.
By contrast with this, a position detecting apparatus is discussed which detects an end part position in a sheet width direction orthogonal to a sheet conveying direction using a photointerrupter in which light emitting elements and light receiving elements are arranged to oppose to each other (Japanese Patent Laid-Open No. 5-132193). The position detecting apparatus discussed in Japanese Patent Laid-Open No. 5-132193, moves a photointerrupter such that a sheet end part crosses between pairs of light emitting elements and light receiving elements arranged to oppose to each other, and detects an end part position of a sheet based on the distance from a reference position to an optical path blocking position.
However, the position detecting apparatus discussed in Japanese Patent Laid-Open No. 5-132193 detects a position by means of pairs of light emitting elements and light receiving elements, and needs to increase the moving distance of the photointerrupter to measure sheet end parts of a plurality of sheet sizes. Further, to secure the moving distance of the photointerrupter, for example, a moving mechanism needs to be made larger. Therefore, the apparatus becomes larger.